Use case: def expect[ReturnToken: Bool = False](mut self, t: Token.Type) -> Token if ReturnToken else None: ref cur = self.current() if unlikely(cur.type != t): raise Error("Unexpected token type " + Token.type_name(cur.type) + " (expected " + Token.type_…

There is now a way in the latest nightlies to do this modular/mojo/stdlib/std/utils/type_functions.mojo at main · modular/modular · GitHub

Thanks, I tried def expect[ReturnToken: Bool = False](mut self, t: Token.Type) -> ConditionalType[If=ReturnToken, Then=Token, Else=NoneType]: ref cur = self.current() if unlikely(cur.type != t): raise Error("Unexpected token type " + Token.type_name(cur.type) + …

Hmmm seems like there may be some compiler limitations around using ConditionalType as a return type. This is the only way I could get this to work right now. from std.utils.type_functions import ConditionalType comptime Cond[b: Bool] = ConditionalType[ Trait=ImplicitlyCopyable, If=b, Then=In…

Thing I noticed: 13 people clicked on the link the last seven hours on Saturday…

When @bgreni talks about the comptime-arts, we listen! I thought I followed nightly stuff pretty well, but I totally missed ConditionalType.

[image] bgreni: ImplicitlyCopyable Thanks Brian, that does seem to be the best possible option for now. In my project, I’ve decided to switch back to 2 separate methods for now - switching to nightly results in compatibility issues with def raises and third party libraries (EmberJson). But i…

FYI: Add draft proposal: Comptime parameterized return type syntax by jbemmel · Pull Request #6178 · modular/modular · GitHub

To vary the return type of a function you can just use a parameter for the actual return type. Then inside the function assign to the parameterised return type using rebind so that the assignment of a concrete type to the parameterised return type works (the types are different names but boil down …

Interesting variation, thanks for sharing

[image] jbemmel: For the Mojo team: Would it be possible to enable the more intuitive syntax like I started with above? Definitely doable, the only reason we need a ConditionalType ATM is that mojo compiler does not know how to merge two type to a common trait bound. We might even be able to…

Comptime parameterized return type

Mojo

jbemmel (Jeroen van Bemmel) March 19, 2026, 10:41pm 12

See [Feature Request] [compiler] Support for Comptime-Parameterized Return Types · Issue #6188 · modular/modular · GitHub

How to resolve unresolved parameter?

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`obj[...]` syntax is "legacy" and should be removed Mojo	20	508	March 19, 2026
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Comptime parameterized return type

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